Corynespora leaf fall disease (Corynespora cassiicola) has caused damage on several rubber clones in the rubber producing countries. Some facts have shpwn that C. cassiicola could reproduce new races in about 10 years and it break down the resistance of rubber clones. These races and its virulence are most various. Virulence of an isolate from a rubber clone from a region varied with differential clones or on the same clone origin of own isolate. A large number of isolates fom rubber clones from the region were virulent on GT 1 and RRIM 600; several of isolates were virulent on BPM 24 and PR 261; and a small number of isolates were virulent on BPM 1, PR 255, PB 260, RRIM 712 and RIC 100, 8 days after inoculation culture of isolates. Isolate from GT 1 from Bengkul was virulent on BPM 24, GT 1 and IAN 873, and isolate from RRIM from South Kalimantan was virulent to GT 1 and IAN 873, 12 days after spraying conidia. Isolate from PR 261 from Bengkulu was virulent on IAN 873, 12 hours after dipping immature leaf in toxin cassiicolin. The evaluation on incidence of disease or virulence of pathogen needs to be carried out from year-to-year. In the fields, the incidences of CLF disease are observed on crown of immature and mature plant or leaf whorl of wood stock and immature plants (more than 10 samples plant per clone per region). In the field experimental and laboratory, virulence of C. cassiicola could be studied using several methods: inoculation culture media or spraying conidia on the leaf or dipping of leaf in toxin cassiicolin. Criteria of assessment incidence of disease or virulence of pathogen have been prepared.